JP4761258B2 - Optical fiber characteristic measuring apparatus and optical fiber characteristic measuring method - Google Patents

Description

  The present invention relates to an optical fiber characteristic measuring apparatus and an optical fiber characteristic measuring method, and more particularly, to optically sense strain distributed to an optical fiber by utilizing a stimulated Brillouin scattering phenomenon that occurs in an optical fiber as an object to be measured. The present invention relates to a fiber characteristic measuring apparatus and an optical fiber characteristic measuring method.

  Brillouin scattering that occurs in an optical fiber varies with strain applied to the optical fiber. Utilizing such a phenomenon, a technique for distributingly measuring strain along the length direction of an optical fiber has been constructed. This measurement technique can measure the magnitude of distortion by measuring the frequency change of the Brillouin scattered light, and also identifies the distortion point of the optical fiber by measuring the time until the Brillouin scattered light returns. Therefore, it is possible to know the distribution of strain applied to structures such as bridges, piers, buildings, dams, and other materials such as aircraft wings and fuel tanks. be able to. Such strain distribution can be used to understand deterioration and aging of structures and materials, and can be used for disaster prevention and accident prevention.

  The strain distribution measurement method known so far is a method in which an optical pulse is incident on an optical fiber and Brillouin scattered light scattered backward is measured in a time-resolved manner. However, in the time domain measurement method using such an optical pulse, the measurement time (which takes several minutes to 10 minutes) is long, and the spatial resolution (1m is the limit) is limited, so various structures can be managed dynamically. This is insufficient for such applications. For this reason, there has been a demand for a breakthrough technique with high spatial resolution and capable of specifying a location where distortion occurs in a shorter time.

  In order to meet these demands, the inventors of the present invention disclosed in Patent Documents 1 and 2 differed from the conventional time-resolved measurement method of optical pulses in that the length direction of the optical fiber is controlled by controlling the interference state of continuous light. Proposed a Brillouin scattering distribution measurement technique along the line and obtained a patent. This technique is known as BOCDA (Brillouin Optical Correlation Domain Analysis) method, and has achieved a spatial resolution of 1 cm and a sampling speed of about 60 Hz, and has attracted attention.

  Here, the principle of Brillouin scattering will be described. When light is incident on a general optical fiber, the wavelength of the ultrasonic wave generated by the thermal oscillation of the glass molecules of the optical fiber material is half the incident light wavelength. An ultrasonic wave is generated. The periodic refractive index change of the glass caused by this ultrasonic wave acts as a Bragg diffraction grating on the incident light, and reflects the light backward. This is the Brillouin scattering phenomenon. The reflected light undergoes a Doppler shift depending on the speed of the ultrasonic wave, but this frequency shift amount changes due to the stretching strain applied to the optical fiber, so that the strain can be detected by measuring the shift amount.

More specifically, as shown in the principle diagram of FIG. 22, from the light source 101 including the semiconductor laser and the signal generator, two propagation waves having different frequencies, that is, strong pump light and weak probe, are measured in the measured optical fiber FUT. Propagate light in opposition. At this time, if a special phase (frequency) matching state is satisfied between the pump light and the probe light (f _pump = f _probe + ν _B : f _pump is the center frequency of the pump light, and f _probe is the center frequency of the probe light. , Ν _B is the Brillouin frequency), and acoustic phonons that scatter photons from the pump light to the probe light are generated by the interaction of both waves. This results in amplification of the probe light as stimulated Brillouin scattering. However, the induction is suppressed when the frequency difference between the pump light and the probe light fluctuates greatly.

As described in Patent Document 1 and the like, the basic principle of the BOCDA method is that the same frequency modulation is applied to the pump light and the probe light propagating in opposition to each other along the optical fiber FUT to be measured. Thus, the stimulated Brillouin scattering having a strong and sharp correlation peak depending on the position is generated periodically. Therefore, in the BOCDA method, the light output from the light source 101 is continuously oscillated light, and the difference between the center frequency f _{probe of the probe} light and the center frequency f _pump of the pump light is changed while the oscillation frequency is changed by a sinusoidal repetitive waveform. The center frequency f _probe of the probe light is changed by an optical frequency converter (not shown) so as to be close to the Brillouin frequency ν _B. As a result, the phase of the pump light and the probe light is asynchronous, and in most positions where the correlation between the two lights is low, the scattered light is spread over a wide frequency range and thinned, while the phases of the pump light and the probe light are synchronized. However, in a narrow region (correlation position) of about a special cm in which the correlation between both lights is high, stimulated Brillouin scattering that forms the original peak spectrum occurs. Then, by moving the correlation position, it is possible to measure the distribution of distortion due to stimulated Brillouin scattering.

  FIG. 22 shows the spectral shape of stimulated Brillouin scattering that occurs at each position of the optical fiber FUT to be measured. Here, BG means Brillouin gain, and Δν means a frequency difference between pump light and probe light. The frequency-modulated light from the light source 101 causes the stimulated scattering spectrum to spread and thin on the frequency axis at most positions of the optical fiber FUT to be measured, but at a special position (correlation position), pump light and probe light. The relative frequency difference from the above becomes constant, and stimulated Brillouin scattering of the original Lorentz spectrum occurs.

Moreover, given sinusoidal frequency modulation with respect to the light source 101 as described above, d _m (distance between adjacent correlated position) the spatial resolution Δz and the measurement range of the BOCDA method by an equation 1 and equation 2 Given each.

Here, V _g is the velocity of light in the optical fiber to be measured FUT, .DELTA..nu _B is the Brillouin gain linewidth of the measured optical fiber FUT (a typical fiber ~30MHz), f _m is the light source 101 The frequency modulation frequency, and Δf is the amplitude of the frequency modulation. As described in Patent Document 1, because only one is present a position where the correlation peak occurs (correlation position) within the measurement target, adjust the measurement range d _m by using Expression 2 To do. In this case, since the measurement range d _m is inversely proportional to the frequency modulation frequency f _m, by lowering the frequency modulation frequency f _m is the speed of frequency modulation of the light source 101, if the frequency changes slowly, zero-order- n the following can increase the space thus measurement range d _m of the adjacent correlation position. However, by simply varying the frequency modulation frequency f _m is 0-order correlation position where the optical path difference is zero between the probe light and the pumping light does not change. Therefore, in order to all of the zero-order ~n order correlation position while maintaining the measurement range d _m changes, may be inserted optical delay in one of the optical path either probe light or the pump light. Thus, by varying the frequency modulation frequency f _m of the light source 101, it is possible to change the correlation position to measure the Brillouin gain spectrum.

In order to increase the measurement range d _m, the thus lowering the frequency modulation frequency f _m, in turn, as is clear from Equation 1, becomes a large value deteriorates the spatial resolution Delta] z. Therefore, while expanding the measurement range d _m, to maintain high spatial resolution Δz, within each spectrum of the probe light and the pumping light does not match overlap with Li, the frequency modulation of the light source 101 amplitude (modulation amplitude) Delta] f Should be increased.

The BOCDA method is a measurement that detects and records the Brillouin gain BG of the probe light at the end of the optical fiber FUT to be measured while sweeping around the frequency difference (Δν) between the pump light and the probe light corresponding to the Brillouin frequency ν _B approximately. Means. Here, the spectrum shape of Brillouin gain obtained from the end of the measured optical fiber FUT (the output of this measurement method) is the Lorentz spectrum (actual signal) generated at the correlation position and the broad spectrum generated at other positions. It is the sum of the integrated value (noise). That is, this will be explained with reference to FIG. 23. In FIG. 23, the spectrum of each detection output is obtained when the correlation position is not distorted (upper diagram) and when the correlation position is distorted (lower diagram). The shape is shown, and the spectrum shape can be divided into a component of the real signal S1 from the correlation peak and a component of the noise S2 from all the positions other than the correlation position. When a distortion or temperature change is given to the position of the correlation peak, only the actual signal S1 shifts from the original frequency difference Δν as shown in the lower diagram of FIG. That is, the peak of the Brillouin gain spectrum generated at the correlation position represents the distortion at the correlation position as the actual signal S1, and the correlation position is swept by changing the frequency (f _m ) of the frequency modulation of the light source 101. If the peak frequency of the spectrum is specified at each correlation position, the strain distribution along the optical fiber FUT to be measured can be measured.
Japanese Patent No. 3667132 Japanese Patent No. 3607930

The apparatus and method of distribution measurement characteristics such as distortion of the measured optical fiber FUT described above, as the optical fiber to be measured FUT to be measured is long, must spread its measurement range d _m, other than the correlation position Unnecessary components diluted at each position are integrated, and the noise S2 increases. That is, since the level of the noise S2 detected by the measuring means is the sum of the Brillouin gain spectra from all the uncorrelated positions, the peak-to-peak ratio (SNR) between the actual signal and the noise is constant. It decreases as the spread measurement range d _m under spatial resolution Δz of. Therefore, as shown in the lower diagram of FIG. 23 in particular, at the position where the shift frequency of the actual signal S1 due to distortion is large, the signal peak becomes smaller than the level of the noise S2, and it becomes impossible to measure the distribution of distortion. Come.

Thus, components of the noise S2 of the background is a factor at the same time results in degradation of accuracy in terms of distribution measuring characteristics of the measured optical fiber FUT, to limit the measurement range d _m, such unnecessary components It has been desired to suppress the noise S2.

  In view of the above-mentioned problems, the present invention effectively suppresses a noise level obtained by integrating unnecessary components from uncorrelated positions, thereby improving the measurement accuracy and extending a measurement range. It is an object of the present invention to provide a characteristic measuring device and an optical fiber characteristic measuring method.

  In order to achieve the above object, an optical fiber characteristic measuring apparatus according to the present invention includes a light source unit that outputs frequency-modulated light, a frequency shift of output light from the light source unit, and a probe from one end of the optical fiber to be measured. A probe light generating means for making the light incident, pump light generating means for making the output light from the light source unit incident as pump light from the other end of the optical fiber to be measured, and a frequency difference between the pump light and the probe light In the optical fiber characteristic measuring device, the light source unit includes a measuring unit that detects a Brillouin gain of the probe light emitted from the optical fiber to be measured while measuring the characteristic of the optical fiber to be measured. Intensity modulation means for modulating the intensity of the output light is provided in synchronization with the frequency modulation.

  In this case, the intensity modulation means brings the intensity closer to the maximum value as the frequency of the output light approaches the fluctuation center, and reduces the intensity to the minimum value as the frequency of the output light approaches the upper limit and the lower limit. It is preferable that the output light frequency be set to a value larger than the minimum value when the frequency of the output light reaches the upper limit and the lower limit.

  In each of the above configurations, it is preferable that the intensity modulation means is configured by a light intensity modulator. Instead, it is preferable that the intensity modulation means is constituted by an optical filter. Alternatively, it is preferable that the intensity modulation means is constituted by a signal generator that frequency modulates the output light from the light source unit with a repetitive waveform other than a sine wave.

  In the optical fiber characteristic measuring method according to the present invention, the light frequency-modulated by the light source unit is frequency-shifted, incident as probe light from one end of the optical fiber to be measured, and the light frequency-modulated by the light source unit The Brillouin gain of the probe light emitted from the optical fiber to be measured is detected while being incident as pump light from the other end of the optical fiber and sweeping the frequency difference between the pump light and the probe light, and the measured object In the optical fiber characteristic measuring method for measuring the characteristic of the optical fiber, the output light is intensity-modulated in synchronization with the frequency modulation of the light source unit.

  In this case, the intensity modulation applied to the output light brings the intensity closer to the maximum value as the frequency of the output light approaches the fluctuation center, and as the frequency of the output light approaches the upper limit and the lower limit, It is preferable to bring the intensity close to the minimum value, and it is preferable to set the intensity of the output light to a value larger than the minimum value at the timing when the frequency of the output light reaches the upper limit and the lower limit.

  In each of the above methods, it is preferable that intensity modulation applied to the output light is performed by a light intensity modulator. Instead, the intensity modulation applied to the output light is preferably performed by an optical filter. Alternatively, the intensity modulation applied to the output light is preferably performed by a signal generator that frequency modulates the output light from the light source unit with a repetitive waveform other than a sine wave.

  According to the optical fiber characteristic measuring apparatus in claim 1 and the optical fiber characteristic measuring method in claim 7 of the present invention, intensity modulation is also performed by the intensity modulation means in synchronization with the frequency modulation applied to the light from the light source. Therefore, the intensity of the output light can be reduced or increased at a specific frequency, and the spectrum distribution of the output light can be adjusted appropriately. Therefore, it is possible to accurately measure the peak frequency of the Lorentz type spectrum generated at the correlation peak position by adjusting the noise spectrum shape that spreads on the frequency axis generated at other than the correlation peak position, and widen the measurement range. Can do.

  According to the optical fiber characteristic measuring apparatus in claim 2 and the optical fiber characteristic measuring method in claim 8 of the present invention, the frequency of the output light from the light source is changed near the upper and lower ends of the frequency as the frequency of the output light fluctuates. It is possible to improve the concentration and bias of light intensity. Therefore, the peak of the Lorentz type spectrum can be maintained larger than the peak of the background spectrum, and the value can be measured correctly even when a large distortion is applied.

  According to the optical fiber characteristic measuring apparatus in claim 3 and the optical fiber characteristic measuring method in claim 9 of the present invention, the Lorentz-type spectrum can be obtained even when the optical fiber under measurement has no or little distortion. The peak can be made larger than the peak of the background spectrum, and accurate strain measurement can be performed regardless of the amount of strain on the optical fiber FUT to be measured.

  According to the optical fiber characteristic measuring apparatus of claim 4 and the optical fiber characteristic measuring method of claim 10 of the present invention, the light intensity modulator receives the synchronization signal from the light source, and the light intensity modulator is suitable for the output light from the light source. Modulation can be performed.

  According to the optical fiber characteristic measuring device of claim 5 and the optical fiber characteristic measuring method of claim 11 of the present invention, the intensity of the output light can be adjusted according to the frequency by the filtering characteristic of the optical filter itself. Therefore, the synchronization signal from the light source is not required, and noise can be reduced and the measurement range can be extended very easily.

  According to the optical fiber characteristic measuring apparatus in claim 6 and the optical fiber characteristic measuring method in claim 12 of the present invention, it is only necessary to change the frequency modulation waveform of the output light to something other than a sine wave using a signal generator. Similarly to the case where the output light is intensity-modulated, the spectral distribution of the output light can be adjusted appropriately, and noise can be reduced and the measurement range can be extended.

  Hereinafter, preferred embodiments of an optical fiber characteristic measuring apparatus and an optical fiber characteristic measuring method according to the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows an optical fiber characteristic measuring apparatus according to a first embodiment of the present invention. In the figure, reference numeral 1 denotes a light source, which is composed of a signal generator 2 and a semiconductor laser 3. As the semiconductor laser 3, for example, a distributed feedback laser diode (DFB LD) that emits a laser beam that is small and has a narrow spectral width is used. The signal generator 2 outputs a desired modulation signal as an injection current to the semiconductor laser 3 in order to frequency-modulate (including phase modulation) the continuous laser light emitted from the semiconductor laser 3 in a sinusoidal manner. It is.

  Reference numeral 4 denotes an optical intensity modulator (IM) as intensity modulation means for intensity-modulating the output light from the semiconductor laser 3 in synchronization with the frequency modulation applied to the semiconductor laser 3. The light intensity modulator 4 here has a function of receiving the synchronizing signal from the signal generator 2 corresponding to the input signal and modulating the intensity of the output light from the semiconductor laser 3. Realized by an optical modulator (EOM). In the present embodiment, it is noticed that such an optical intensity modulator 4 is added to an existing optical fiber characteristic measuring apparatus using the BOCDA method, but another configuration of the intensity modulating means will be described later.

  Reference numeral 6 denotes a first optical branching unit that bisects the laser light whose frequency and intensity are modulated by the light intensity modulator 4 into, for example, an intensity ratio of 90/10. One of the branched laser lights is doped with erbium. Amplified by an optical fiber amplifier (hereinafter referred to as EDFA) 7. Further, the intensity-modulated light amplified by the EDFA 7 is lowered in frequency by about 10 GHz by a single sideband modulator (SSBM: SSB modulator) 8 and enters one end of the optical fiber FUT to be measured as probe light.

  The SSB modulator 8 uses microwaves and accurate DC bias control so as to suppress high frequency components in the two primary sidebands and maintain a stable frequency difference Δν from the pump light. In addition, a low-frequency sideband having a frequency difference Δν equal to the microwave frequency with respect to the input light is output as probe light. Further, a polarization switch (PSW) 9 for applying the polarization diversity method is inserted after the SSB modulator 8. This polarization switch 9 is provided in order to suppress shake depending on the polarization of the Brillouin gain.

  On the other hand, the other laser beam branched by the first optical branching unit 6 passes through an optical delay unit 11 made of an optical fiber having a predetermined length, and is transmitted by an optical intensity modulator 13 having a reference signal generator 12. After intensity modulation, it is amplified by EDFA14. The intensity-modulated light amplified by the EDFA 14 passes through the second optical splitter 15 and then enters as pump light from the other end of the measured optical fiber FUT. The probe light and the pump light enter the measured optical fiber FUT. Are propagating face-to-face. As described above, the optical delay device 11 is for setting a predetermined delay time between the pump light and the probe light, and the delay time can be appropriately adjusted by changing the optical fiber length. .

  Light emitted from the other end of the optical fiber FUT to be measured is taken into the photodetector 20 via the second optical branching device 15, and its power is measured. The detection output from the photodetector 20 is synchronously detected at the modulation frequency of the pump light by passing through the lock-in amplifier 21, and the Brillouin gain of the probe light accompanying the induced Brillouin phenomenon is a measuring means configured by, for example, a personal computer The data processor 23 receives the final data at a predetermined sampling rate.

  The light intensity modulator 13 provided in the optical path of the pump light is constituted by, for example, an electro-optic modulator, like the light intensity modulator 4. The first optical branching device 6 and the second optical branching device 15 may use a circulator, a beam splitter, a half mirror, or the like. As yet another modification, the light source 1 as the light source unit may be provided with a separate light source for each of the probe light and the pump light. In that case, the intensity synchronized with the frequency modulation of each light source 1 The modulation means may be provided for each light source 1.

  In this embodiment, the EDFA 7, the SSB modulator 8, and the polarization switch 9 constitute probe light generation means 31 that generates probe light from the output light of the light source 1, and includes an optical delay device 11, an optical intensity modulator 13, and an EDFA 14. The second optical splitter 15 constitutes pump light generating means 32 for generating pump light from the output light of the light source 1, and the photodetector 20, lock-in amplifier 21, and data processor 23 are composed of the pump light and the probe. Measuring means 33 that detects the Brillouin gain of the probe light emitted from the end of the optical fiber FUT to be measured while sweeping the frequency difference with the light, and measures distortion that is one of the characteristics of the optical fiber FUT to be measured. It is composed.

As described above, according to the basic principle of the BOCDA method, the apparatus shown in FIG. 1 uses continuous light as light from the light source 1 and changes its oscillation frequency by a signal generator 2 in a sinusoidal repetitive waveform. The SSB modulator 8 changes the center frequency f _probe of the probe light so that the difference between the center frequency f _probe of the probe light and the center frequency f pump of the pump light is _close to the Brillouin frequency ν _B. As a result, the phase of the pump light and the probe light is asynchronous and the induction is suppressed at most positions where the correlation between the two lights is low, but at the correlation position where the phases of the pump light and the probe light are synchronized, the stimulated Brillouin scattering is performed. Occurs. Then, by moving this correlation position, it becomes possible to measure the distribution of distortion due to Brillouin scattering.

The correlation position by stimulated Brillouin scattering is periodic along the fiber FUT to be measured sandwiched between the isolator 16 and the circulator (second optical splitter) 15 because the modulation to the probe light and the pump light is periodic. Appear in Therefore, as there is only a correlation peak one positionally within the fiber to be measured FUT, adjusting the frequency modulation frequency f _m applied to the delay amount and the semiconductor laser 3 of the optical delay device 11. Further, in order to widen the measurement range while maintaining the spatial resolution Δz as a high device, the amplitude Δf of the frequency modulation for the semiconductor laser 3 is set within a range where the spectrums of the probe light and the pump light do not overlap. Need to increase.

Next, the operation of the apparatus shown in FIG. 1 will be described. The laser light frequency-modulated from the semiconductor laser 3 by the injection current from the signal generator 2 is emitted, and the light intensity modulator 4 is emitted from the signal generator 2. The output light passing through the light intensity modulator 4 is intensity-modulated in synchronism with the frequency modulation. The laser light whose frequency and intensity are both modulated is branched into a predetermined intensity ratio by the first optical branching device 6 , and one light is amplified by the EDFA 7 and then input to the SSB modulator 8. The SSB modulator 8 performs SSB modulation on the modulated light and generates a sideband having a frequency difference Δν (about 10 GHz) close to the Brillouin frequency ν _B with respect to the center frequency of the modulated light. 9 and the isolator 16 and is incident as one probe light on one end of the optical fiber FUT to be measured.

  On the other hand, the other modulated light branched from the first optical branching device 6 passes through the optical delay device 11 and is given a predetermined delay time, and is then input to the light intensity modulator 13, where a reference signal is generated. The intensity is modulated based on the frequency of the reference signal generated from the device 12. The modulated laser light chopped by this intensity modulation is amplified by the subsequent EDFA 14, passes through the second optical splitter 15, and enters the other end of the measured optical fiber FUT as pump light.

  Thus, when the probe light and the pump light propagate in opposite directions in the measured optical fiber FUT, a part of the pump light that has been reflected or backscattered is emitted from the measured optical fiber FUT and stimulated Brillouin scattering. The increase in the probe light due to is superimposed on the continuous probe light and emitted from the measured optical fiber FUT. When these emitted lights are detected by the photodetector 20, and synchronous detection is performed at the intensity modulation frequency of the pump light by the lock-in amplifier 21, only the increase in the probe light generated in synchronization with the chopping of the pump light is extracted and amplified. It is output and other frequency components are removed. Although not shown in FIG. 1, a part of the pump light that has undergone reflection or backscattering is also emitted from the optical fiber FUT to be measured. An optical filter may be interposed before the 20th stage. The data processor 23 receives the output signal from the lock-in amplifier 21, determines the frequency at which the peak of the stimulated Brillouin scattering spectrum at the correlation peak position is, and measures the distribution of distortion in the measured optical fiber FUT. Do.

Here, in the conventional device not provided with the light intensity modulator 4 as is, if you widen the measurement range d _m under certain spatial resolution Delta] z, and the actual signal S1 by the correlation peak shown in FIG. 23, the non The peak-to-peak ratio (SNR) with respect to the noise S2 that accumulates the thinned unnecessary components from the correlation position becomes small, and the peak frequency of the actual signal S1 cannot be measured correctly. This is because the spectral distribution of the output light from the light source 1 cannot be arbitrarily adjusted.

In this regard, in the present embodiment, not only frequency modulation is performed on the output light from the light source 1 but also intensity modulation is performed in synchronization with this frequency modulation by the light intensity modulator 4. It is also possible to weaken or increase the intensity of the output light at a specific frequency, and the spectral distribution of the output light can be appropriately adjusted so as not to be emphasized only in a specific frequency region. Therefore, to improve the SNR between the real signal S1 and the noise S2 by the correlation peak by reducing the noise S2, while correctly measure the peak frequency of the real signal S1, it is possible to increase the measurement range d _m.

  Next, an experimental example of the apparatus in FIG. 1 and the results thereof will be described. In this experimental example, a 1552 nm distributed feedback laser diode (DFB LD) is used as the semiconductor laser 3 of the light source 1, and a signal generator 2 is used to generate a correlation peak in the test optical fiber FUT. A sinusoidal frequency modulation was given. The output from the semiconductor laser 3 was directly used as Brillouin pump light, and was amplified by the EDFA 14 after passing through a 3 km delay fiber as an optical delay device 11 in order to control the order of the correlation peak. On the other hand, the probe light suppresses the higher frequency component of the two primary sidebands, and in order to obtain a stable frequency difference Δν with respect to the pump light, SSB modulation using microwaves and accurate DC bias control Was generated through a vessel 8. The SSB modulator 8 has a suppression ratio of 25 dB or more with other frequency components. The pump light was intensity modulated for lock-in detection, and its chopping frequency was 3.8 MHz. A photodiode with a bandwidth of 125 MHz was used as the photodetector 20, and the final data was captured after the lock-in amplifier 21.

Frequency modulation frequency f _m of the semiconductor laser 3 is a 310~330kHz depending on the correlation peak position of the measured optical fiber FUT, which according to the number 2, the interval of more correlation peaks 310m i.e. measurement range d corresponding to _m . The frequency modulation amplitude Δf is 9.5 GHz, and from Equation 1, the spatial resolution Δz of the measurement is calculated to be about 30 cm.

As shown in FIG. 5, the optical fiber FUT to be measured is a fusion of a continuous general fiber (SMF: single mode optical fiber) and three dispersion-shifted optical fibers (DSF) having a length of 30 cm. The total length is about 305m. The average Brillouin shift amount of the optical fiber FUT to be measured, that is, the Brillouin frequency νB, was 10.5 GHz in the DSF part and 10.8 GHz in the SMF part. The difference in Brillouin frequency ν _B in these two fiber sections corresponds to an induced strain of ˜6000 με that can cover most situations of distributed sensing.

  In this experimental example, FIG. 6A shows a case where only conventional sinusoidal frequency modulation is applied without applying intensity modulation, and a case where three different intensity modulations are applied using the light intensity modulator 4. Modulated light with different power spectrum as shown in Fig. 2 was generated. In the figure, “No IM” is modulated light when only conventional sine wave frequency modulation is applied, and “IM1,” “IM2,” and “IM3” are intensity modulated by the light intensity modulator 4. This means modulated light, and for each modulated light, a time-average power spectrum measured by an optical spectrum analyzer is shown.

  Here, the power spectrum of the modulated light (IM1) having a flat upper portion over the most frequency range is generated by first compensating the power spectrum of the conventional modulated light (No IM). The power spectrum of the other modulated light (IM2, IM3) was generated by adjusting the offset and amplitude applied to the light intensity modulator 4 when obtaining the power spectrum of the modulated light (IM1). That is, since the frequency of modulated light (No IM) obtained from the conventional light source 1 is sine wave-shaped, it stays for a relatively long time at the maximum displacement portion of the frequency, and the power spectrum is near the upper and lower limits of the modulated frequency. Strength increases at both ends. On the other hand, in the modulated light (IM1), the intensity is adjusted so as to be substantially flat over the entire frequency width by applying the light intensity modulation in synchronization. Also, with another modulated light (IM2), the intensity is adjusted so that the intensity is maximum at the center of the frequency width (that is, has a convex intensity characteristic with respect to the frequency), and further, the modulated light (IM3) In this case, the intensity is adjusted so as to increase at substantially the center of the frequency width while leaving portions where the intensity increases at both ends of the frequency width. In FIG. 6B, a modulation voltage indicating a conventional frequency modulation waveform (solid line) and a light intensity modulator 4 provided to generate a power spectrum of modulated light (IM1) having a flat upper portion are shown. The transmittance (broken line) is shown.

  The Brillouin gain spectrum measured using each modulated light (No IM, IM1, IM2, IM3) is measured at each part of the measured optical fiber FUT, and the “2” part of the DSF in FIG. The part in position was selected as the measurement point. Here, the frequency difference Δν of the SSB modulator SSBM was swept from 10.2 GHz to 11.2 GHz, and the overall measurement speed for one point was 10 Hz.

  FIG. 7 shows Brillouin gain spectra measured in the DSF unit and the SMF unit by the measuring means 33 using the modulated lights (No IM, IM1, IM2, IM3) generated in this way. In the figure, the upper left graph shows the Brillouin gain spectrum of the DSF part and SMF part by modulated light (No IM) that is not subjected to conventional intensity modulation, but in the DSF part, the actual signal component is the noise level. Here, the state of the actual signal cannot be accurately detected, and it becomes impossible to measure the distortion amount by final peak frequency determination. On the other hand, the upper right, lower left, and lower right graphs show the Brillouin gain spectra of the DSF part and SMF part for each modulated light (IM1, IM2, IM3) subjected to intensity modulation. In all cases, it is observed in the DSF part that the noise peak is considerably reduced compared with the amplitude of the actual signal. Therefore, in any case of each modulated light (IM1, IM2, IM3), the DSF unit can correctly determine the peak frequency. In the case of modulated light (IM1, IM2), a large dent is observed at the center of the background noise level, and this causes the “absorption” effect of the actual signal as shown in the Brillouin gain spectrum BGS of the SMF part. This causes a problem that the Lorentz spectrum cannot be observed when measuring the part. However, such depressions can be removed by controlling the intensity modulation offset and amplitude while maintaining the overall shape of the Brillouin gain spectrum BGS. The observation result of the modulated light IM3 shows an optimal situation in this respect. Here, the background noise level of the Brillouin gain spectrum BGS remains low and flat with respect to the actual signal in both the SMF part and the DSF part. The signal peaks are clearly distinguished with respect to the noise level. That is, the modulated light IM3 here is an example of the most effective intensity modulation in that the peak frequency of the actual signal can be correctly determined in any part.

Under the same signal amplitude, if not subjected to intensity modulation (No IM) and the optimal intensity when the modulation subjected to (IM3) and Buriruangei Nsu spectrum measurement result of comparison is shown in Figure 8 Yes. In the figure, (a) shows the measurement result of the DSF part, and (b) shows the measurement result of the SMF part. When the SNR is defined as a peak-to-peak ratio between the actual signal and the background noise, in the Brillouin gain spectrum in the DSF portion, the SNR is changed from 0.82 to 1.27, and an improvement of 45% is calculated. Considering the achieved SNR, this experimental apparatus seems to have a margin for further expanding the measurement range dm.

In order to confirm the effect of modulated light (IM3) with optimal intensity modulation, using the same experimental equipment, the measurement optical fiber FUT can be used with or without intensity modulation. FIG. 9 shows the results of distribution measurement in units of 10 cm. Here, in correspondence with “1”, “2”, and “3” in FIG. 5, the relationship between each position of the DSF portion and the Brillouin frequency ν _B is shown in (a), (b), and (c). Each is shown. As clearly shown in FIG. 9, in the modulated light (IM3) subjected to optimum intensity modulation, the DSF part at any position is accurately detected, and intensity modulation is applied to this. If not (NO IM), the position as the DSF section is lost. Here, the measurement error of the Brillouin frequency ν _B was about ± 3 MHz.

  As described above, a novel apparatus and method for improving the performance of the BOCDA method by performing intensity modulation on the output light from the light source 1 has been demonstrated. According to this experimental result, by applying appropriate intensity modulation, the background noise level can be changed and suppressed in the Brillouin gain spectrum, and the measurement range can be greatly increased. In addition, 30cm spatial resolution and 300m distribution measurement became possible, and even when normal frequency modulation alone could not measure accurately, improved performance could be confirmed. As a result, the SNR was improved to over 40%.

Next, another experimental example obtained by further optimizing the modulation waveform and the result will be described. Experimental apparatus used is the same as that shown in FIG. 1, the amplitude Δf of the frequency modulation and 32.5GHz, also the frequency modulation frequency f _m of the semiconductor laser 3 was 91～101KHz. This corresponds to a measurement range dm of 1010 _m or more and a spatial resolution Δz of less than 30 cm. In addition, a delay fiber of 10 km or more was used as the optical delay device 11.

  Furthermore, the optical fiber FUT to be measured here has three general fibers (SMF) each having a length of 100 m, 400 m and 500 m, and four each having a length of 30 cm, as shown in FIG. It is connected to a dispersion shifted optical fiber (DSF), and its overall length is about 1010m. The average Brillouin frequency νB of the optical fiber FUT to be measured is 10.5 GHz in the DSF part and 10.8 GHz in the SMF part, which is the same as that shown in FIG.

  In this experimental example, the conventional intensity modulation is not performed and only the sine wave frequency modulation is applied (No IM), and the optimum intensity modulation is applied using the light intensity modulator 4 (with IM). For the above, modulated light having different power spectra as shown in FIG. 11 was generated. In this case as well, the power spectrum of the modulated light (No IM) obtained without intensity modulation increases in intensity at both ends near the upper and lower limits of the modulated frequency, and the optimum modulated light (with IM) The power spectrum is adjusted so that the intensity is increased even at substantially the center of the modulated frequency width while leaving portions where the intensity is increased at both ends of the modulated frequency.

FIG. 12 shows a comparison result of the Brillouin gain spectrum measured in the conventional case (No IM) and when the optimum intensity modulation is performed (with IM). 12 (a) and 12 (b) show the Brillouin gain measurement results when the frequency difference Δν of the SSB modulator SSBM is swept from 10.2 GHz to 11.2 GHz in each of the DSF unit and the SMF unit. . The peak of the Brillouin frequency ν _B indicated by an arrow is a local Brillouin signal (the above-mentioned real signal) from the correlation peak, and its amplitude is normalized on the same scale. The remaining part is accumulated background noise from all remaining parts of the measurement optical fiber FUT. In both the DSF part and SMF part of the measurement optical fiber FUT, when the optimum intensity modulation is performed (with IM), the background noise amplitude is larger than when the conventional intensity modulation is not performed (No IM). The comparison clearly shows a significant reduction. In particular, in the measurement result of the DSF part in FIG. 12 (a), when optimum intensity modulation is performed (with IM), the noise level is suppressed lower than the Brillouin signal, and the local Brillouin frequency by simple peak search is suppressed. Can be detected accurately.

FIG. 13 shows the results of distribution measurement in units of 10 cm on the measurement optical fiber FUT when optimum intensity modulation is performed (with IM). Here, corresponding to “1”, “2”, “3”, and “4” in FIG. 10, the relationship between each position of the DSF portion and the Brillouin frequency ν _B is (a), (b), They are shown in (c) and (d), respectively. As is clear from the results of FIG. 13, a DSF portion of 30 cm is correctly detected over the measurement optical fiber FUT having a length of 1 km. Here, the measurement error of the Brillouin frequency ν _B was about ± 3 MHz.

  As described above, in this experimental example, by applying appropriate intensity modulation to the output light from the light source 1 by the light intensity modulator 4 so that light having a spectrum shape as shown in FIG. 11 is emitted, The performance of the BOCDA method can be further enhanced. According to this experimental result, by applying appropriate intensity modulation, the background noise level in the Brillouin gain spectrum can be further suppressed, and distribution measurement over a range of 1 km can be performed with a spatial resolution of 30 cm.

  As described above, the optical fiber characteristic measuring apparatus according to the first embodiment includes the light source 1 as a light source unit that outputs frequency-modulated light, and SSB modulation that uses a part of the output light from the light source 1 as frequency shift means. The probe 8 generates a probe light from the optical fiber FUT, and the remaining part of the output light from the light source 1 is pumped from the other end of the optical fiber FUT. The Brillouin gain of the probe light emitted from the measured optical fiber FUT is detected while sweeping the frequency difference Δν between the pump light and the probe light, and the pump light generating means 32 that is made incident as light, and the measured optical fiber FUT In the apparatus provided with the measuring means 33 for measuring the characteristics, the light source 1 is applied as a spectral distribution adjusting means for arbitrarily adjusting the spectral distribution with respect to the frequency of the light from the light source 1. The light intensity modulator 4 is provided as intensity modulation means for modulating the intensity of the output light in synchronization with the frequency modulation.

  Correspondingly, in the optical fiber characteristic measuring method in the first embodiment, the frequency-modulated light from the light source 1 which is a light source unit is frequency-shifted by, for example, the SSB modulator 8, etc., and the optical fiber FUT to be measured While being incident as probe light from one end, light that is similarly frequency-modulated by the same or another light source 1 is incident as pump light from the other end of the optical fiber FUT to be measured, and the frequency difference between the pump light and the probe light. In the method of detecting the Brillouin gain of the probe light emitted from the measured optical fiber FUT while sweeping Δν and measuring the characteristics of the measured optical fiber FUT, the spectral distribution with respect to the frequency of the light from the light source 1 is arbitrarily set. In order to adjust, intensity modulation is performed on the output light in synchronization with the frequency modulation of the light source unit.

  Note that the light source section here is not only one that generates probe light and pump light from a single light source 1 as shown in FIG. 1, but also one that has a light source for each of probe light and pump light. Including.

In the above apparatus and method, since the intensity modulation is also performed by the intensity modulation means in synchronization with the frequency modulation given to the light from the light source 1, the intensity of the output light is weakened or increased at a specific frequency. And the spectral distribution of the output light can be appropriately adjusted. Therefore, by adjusting the noise spectrum shape spread on the frequency axis generated by the non-correlation peak position, the peak frequency of the Lorentzian it becomes possible to measure accurately generated at the correlation peak position, the measurement range d _m Can be spread. That is, by effectively suppressing the noise level by integrating the unwanted components from the non-correlation position, with improved measurement accuracy, the novel optical fiber characteristic measuring device and the light can be extended measurement range d _m A fiber characteristic measuring method can be provided.

  Further, in the optical fiber characteristic measuring apparatus according to the present embodiment, the intensity modulation means is constituted by the light intensity modulator 4. Correspondingly, in the optical fiber characteristic measuring method in the present embodiment, intensity modulation applied to output light is performed by the light intensity modulator 4.

  In this case, the light intensity modulator 4 can perform appropriate intensity modulation on the output light from the light source 1 in response to the synchronization signal from the light source 1.

  Next, several preferred embodiments will be described for another apparatus and method that replaces the first embodiment. In addition, the same code | symbol is attached | subjected to a common part with 1st Embodiment, and description of the same location is abbreviate | omitted as much as possible in order to avoid duplication.

FIG. 2 shows an apparatus according to a second embodiment of the present invention. Here, instead of the light intensity modulator 4, an optical filter 41 having an appropriate transmission spectrum characteristic is replaced with an optical path of output light from the light source 1. Arranged inside. Also in this case, in synchronization with the frequency modulation of the output light from the light source 1, the optical filter 41 as the intensity modulation means substantially performs the intensity modulation, and the spectral distribution of the output light can be adjusted appropriately. In addition, when the optical filter 41 is used, the intensity of the output light can be adjusted according to the frequency due to the filtering characteristics of the optical filter 41 itself, so that a synchronization signal from the signal generator 2 is required. not, it can be realized very easily extended to reduce the measurement range d _m of the noise S2.

  Further, as another configuration of intensity modulation means, instead of the external modulation type light intensity modulator 4 in the first embodiment, a direct modulation type signal that frequency-modulates the output light from the light source 1 with a repetitive waveform other than a sine wave. Generator 51 may be used. FIG. 3 shows an example thereof as a third embodiment. The signal generator 51 here has a function of frequency-modulating the output light from the semiconductor laser 3 with a triangular waveform, for example.

  FIG. 14 shows a frequency modulation waveform when the output light is frequency-modulated with a conventional sine wave-like repetitive waveform and a time-modulated spectrum calculated from the frequency modulation waveform when the output light is frequency-modulated with a repetitive waveform other than a sine wave. Each shape is shown. FIG. 2A shows a conventional frequency modulation waveform (in the example shown here, a cosine wave, but is substantially equivalent) in which the frequency of the output light of the light source 1 is changed in a sine wave form in the upper stage. However, in this case, it stays relatively long at the maximum displacement part of the fluctuating frequency, so the spectrum intensity (power) is large at both ends near the upper and lower limits of the frequency as shown in the waveform of the lower spectrum shape. It will be biased. On the other hand, when the frequency of the output light of the light source 1 is changed in the triangular wave shape shown in the upper part of FIG. 14B, the frequency stays for the same time at any frequency, so as shown in the lower part of FIG. The spectrum intensity becomes uniform. Furthermore, when the frequency of the output light of the light source 1 is changed with the repetitive waveform shown in the upper part of FIG. 14C, the spectrum intensity becomes Gaussian as shown in the lower part of FIG.

FIG. 15 shows the simulation result of the Brillouin gain spectrum shape when the DSF part of the optical fiber FUT to be measured is measured using the output light having each frequency modulation waveform shown in FIG. Appears. In FIG. 14, W1 matches the condition of FIG. 14A, W2 matches the condition of FIG. 14B, and W3 matches the condition of FIG. 14C. In addition, the optical fiber FUT to be measured here assumes that only 1/1000 of the entire optical fiber FUT is a DSF part and the rest is an SMF part. FIG. 15A shows the normalized Brillouin gain spectrum shape, and FIG. 15B shows the absolute Brillouin gain spectrum shape. In particular, the output light from the light source 1 is frequency-modulated with a triangular waveform. In this case, it can be seen that the peak frequency of the DSF part can be easily determined. Thus, by simply modifying the frequency modulation waveform, to the output light of the light source 1 can be subjected to equivalently intensity modulation, it expanded reduce the measurement range d _m of noise is possible to some extent.

  In the third embodiment, the intensity modulation means described above is configured by a signal generator 51 that frequency modulates the output light from the light source 1 with a repetitive waveform other than a sine wave. Here, intensity modulation applied to the output light is performed by a signal generator 51 that frequency modulates the output light from the light source 1 with a repetitive waveform other than a sine wave. In this way, just by changing the frequency modulation waveform of the output light to something other than a sinusoidal waveform using the signal generator 51, the spectral distribution of the output light is appropriately adjusted as in the case where the intensity modulation is performed on the output light. The noise can be reduced and the measurement range can be expanded.

  FIG. 4 shows an apparatus according to a fourth embodiment of the present invention. In this embodiment, the “double lock-in method” previously proposed in Japanese Patent Application No. 2005-348482 by the inventors of the present application is incorporated into the apparatus configuration of the first embodiment. In FIG. 4, the light intensity modulator 4 is used as the intensity modulation means, but the optical filter 41 in the second embodiment and the signal generator 51 in the third embodiment may be used instead. Here, intensity of the probe light is modulated by the second light intensity modulator 62 having the second reference signal generator 61 at a frequency different from that of the pump light, and the detection output from the light detector 20 is By passing through the first lock-in amplifier 21 and the second lock-in amplifier 22 connected in series, synchronous detection is performed at the modulation frequency of the pump light and the modulation frequency of the probe light, respectively, and the increase of the probe light accompanying the induced Brillouin phenomenon Only the amount is taken into the data processor 23 as final data at a predetermined sampling rate.

  Then, the probe light and the pump light propagating through the measured optical fiber FUT are chopped at different frequencies by the second light intensity modulator 62 and the first light intensity modulator 13, respectively. When both such lights are propagated in opposition in the measured optical fiber FUT, they are superimposed on the chopped probe light, and the increase in the probe light due to stimulated Brillouin scattering is chopped by the intensity modulation frequency of the pump light. In this state, the light is emitted from the measured optical fiber FUT. When this emitted light is detected by the photodetector 20 and synchronously detected at the intensity modulation frequency of the pump light by the first lock-in amplifier 21, a part of the pump light including the same intensity modulation frequency component and the increase in the probe light are detected. Are extracted and amplified and output, and other frequency components are removed. A part of the pump light detected by the first lock-in amplifier 21 is not affected by the intensity modulation frequency of the probe light, but is also probe light by stimulated Brillouin scattering detected by the first lock-in amplifier 21. The increase in is synchronized with the intensity modulation frequency of the original probe light. Therefore, when synchronous detection is performed at the intensity modulation frequency of the probe light by the second lock-in amplifier 22 at the subsequent stage, only the increase of the probe light is extracted and amplified and output, and other noise including part of the other pump light The component is now completely removed.

That is, in the apparatus shown in FIG. 4, since the original probe light is intensity-modulated at a frequency different from that of the pump light, the first lock-in amplifier 21 uses a part of the pump light and the increase in the probe light. Even after synchronous detection, only the increase in probe light necessary for distortion measurement is synchronized with the intensity modulation frequency of the original probe light. If this is utilized, the increase in the probe light can be completely separated from the other components by the second lock-in amplifier 22 without arranging the optical wavelength filter in front of the photodetector 20. Moreover, use for the purpose of enlarging the measurement range d _m while maintaining a high spatial resolution Δz as a device, even if the amplitude Δf of the frequency modulation of the light source 1 was spread to some extent, the frequency difference Δν of the pump light and the probe light Since the increase in the probe light is not detected, only the necessary increase in the probe light can be correctly detected without being affected by the amplitude Δf.

Thus, the intensity modulation explained in the first to third embodiments, by combining the "double lock-method" described in this fourth embodiment can realize a synergistically expansion of the measurement range d _m. Of course, in various BOCDA method other than the "double lock-method", also incorporate the concept of intensity modulation explained in the first to third embodiments, the thereby improving the measurement accuracy, the ability to expand the measurement range d _m is Needless to say.

  Next, simulation results of the conventional example and the BOCDA system in the present embodiment will be described with reference to FIGS.

  FIG. 16 shows a simulation waveform of each part of the BOCDA system when only the frequency modulation is performed on the output light from the light source 1. Here, sinusoidal frequency modulation is performed on the output light by the upper waveform in FIG. 16A to localize the position where stimulated Brillouin scattering occurs along the measured optical fiber FUT. On the other hand, the lower part of FIG. 16A shows the intensity modulation waveform of the output light, which indicates that intensity modulation is not performed here. At this time, the time average spectrum of the output light of the light source 1 is as shown in FIG. 16B, which is equal to the waveform in the lower part of FIG.

  In this case, a Lorentz spectrum is generated at a position (measurement point) where stimulated Brillouin scattering is localized. On the other hand, the spectrum spreads on the frequency axis except for the localized position, and the integral along the measured optical fiber FUT appears in the output spectrum of the present measurement system. The spectral shape is a so-called Mt. Fuji type that has a slight depression at the center frequency, and has a slope portion that decreases in a curved shape as the frequency becomes higher or lower. FIG. 16C shows the output spectrum of the BOCDA system obtained when there is distortion at only one location along the measured optical fiber FUT. Here, when the magnitude of the distortion changes, the Lorentz-type spectrum changes so as to slide on the Fuji-type background spectrum.

Conventionally, when the measurement range i.e. measurement range d _m is longer than the spatial resolution Delta] z, the measurement accuracy drops this background spectrum becomes relatively large. More importantly, conventionally, when the amount of distortion increases, the peak of the Mt. Fuji-type background spectrum becomes higher than the tip of the Lorentz-type spectrum, making it impossible to detect the distortion.

FIG. 16D is a plot of the ratio of the spectrum at the measurement point and the height of the Mt. Fuji spectrum as a function of the amount of distortion. When the value of this SNR is 1 or less, distortion measurement becomes impossible. In this simulation, as shown in FIG. 16 (d), it is shown that 230 MHz is the limit of strain measurement as the shift amount of the Brillouin frequency ν _B , which corresponds to the strain amount of 0.46% (4,600με). To do.

  Next, a case where the output light is combined with frequency modulation and intensity modulation will be described. When the semiconductor laser 3 is used as the light source 1 and the frequency modulation is performed by utilizing the direct frequency modulation characteristics as in the above embodiments, the frequency modulation is accompanied by a change in the injection current to the semiconductor laser 3. Intensity modulation also occurs. This effect is simulated in FIG.

  Here, it is considered that both the frequency and the intensity change in proportion to the injection current of the semiconductor laser 3. This is shown in the frequency modulation waveform of the output light in the upper stage of FIG. 17A and the intensity modulation waveform of the output light in the lower stage of FIG. As the intensity modulation for the output light increases, the time average spectrum shown in FIG. 17B becomes asymmetric. For example, when the case where the modulation rate indicating the degree of intensity modulation is gradually increased to 30%, 60%, and 90% is simulated, the asymmetry of the time average spectrum increases. As such asymmetry increases, the slope of the Mt. Fuji-type background spectrum becomes steep as shown in FIG. That is, the detection limit of the distortion amount becomes smaller. Note that “No IM” in FIG. 17C is a spectrum shape when the conventional intensity modulation is not performed.

FIG. 17D is a plot of the ratio of the spectrum at the measurement point and the height of the Mt. Fuji spectrum as a function of the amount of distortion. Here, it is clearly shown that as the asymmetry of the time average spectrum increases, that is, the intensity modulation increases, the detection limit of the distortion amount decreases. As an example, in intensity modulation with a modulation rate of 60%, the strain measurement limit is approximately 200 MHz as the shift amount of the Brillouin frequency ν _B.

  That is, when utilizing the direct frequency modulation characteristic of the semiconductor laser 3, it is desired to compensate for the influence of intensity modulation caused by this.

  FIG. 18 shows a simulation result when intensity modulation is performed in synchronization with frequency modulation changing in a sine wave shape to compensate for the influence of such intensity modulation. When frequency modulation is performed on the output light from the light source 1 with a sine wave, the intensity increases on both sides of the time-average spectrum. In order to compensate for this, it is assumed that any one of the first to fourth embodiments is selected and intensity modulation synchronized with frequency modulation is performed.

  The upper part of FIG. 18A shows the frequency modulation waveform of the output light that changes in a sine wave shape, and the intensity modulation is performed so that the intensity of the output light is minimized in synchronization with the upper and lower limits of the fluctuating frequency. The lower part of FIG. 18A shows an example of such intensity modulation. “Full” represents intensity modulation in which the time average spectrum is flat over the entire frequency. The time average spectrum in this case is shown as “Full” in FIG. Further, “Half” in the lower part of FIG. 18A is a case where the minimum value of intensity modulation is set to 0.5, which is half of the maximum value, and the values on both sides of the time average spectrum are reduced, but are not flat. . FIG. 18B also shows the time-average spectrum at this time (see the waveform of “Half” in the figure).

  The output spectrum of the BOCDA system at this time is as shown in FIG. In the case of “Half” shown in the upper part of FIG. 18C, the height of the Mt. Fuji-type background spectrum is low, and in the case of “Full” shown in the lower part of FIG. 18C, the background spectrum is flat. It has become. However, in the center of the background spectrum, a depression is formed in both cases of “Half” and “Full”. As a result, in “Half”, when there is no distortion (a uniform Brillouin frequency shift amount over the entire length of the optical fiber), the Lorentz spectrum at the measurement point falls into the depression and becomes low. Furthermore, in the case of “Full”, the Lorentz type spectrum completely falls into the depression, and the strain measurement becomes impossible.

  FIG. 18D shows a plot of the ratio of the height of the spectrum at the measurement point and the Mt. Fuji spectrum as a function of the amount of distortion. It can be seen that measurement is difficult when the measurement point has less (or no) distortion than the surroundings. However, on the other hand, even if the distortion becomes large, it is not hidden in the background spectrum, and in this respect, it can be seen that the distortion measurement limit is improved as compared with the measurement systems shown in FIGS.

  In FIG. 18, the intensity approaches the maximum value as the frequency of the output light approaches the fluctuation center, and the minimum value becomes, for example, 0.5 or less as the frequency of the output light approaches the upper limit and the lower limit. Apply “Half” and “Full” intensity modulation. In this way, it can be improved that the intensity of the output light is concentrated and biased near the upper and lower ends of the frequency as the frequency of the output light from the light source 1 fluctuates. Therefore, even if the strain on the optical fiber FUT to be measured increases, the Lorentz-type spectrum peak at the measurement point can be kept larger than the peak of the Mt. Fuji-type background spectrum, and the value can be measured correctly even when a large strain is applied. However, it is difficult or impossible to measure a position with little (or no) distortion.

  Simulations show that an effective means of solving this problem is to leave a little more intensity on both sides of the time average spectrum.

  A waveform showing the relationship between intensity modulation and time for this purpose is shown in the lower left of FIG. The waveform surrounded by the square in the right figure shows the intensity modulation corresponding to both side portions of the output light spectrum. Here, the intensity modulation means brings the intensity closer to the maximum value as the frequency of the output light that changes in a sine wave approaches the fluctuation center, and the intensity as the frequency of the output light approaches the upper limit and the lower limit. In addition to the original “Half” and “Full” intensity modulation that brings the maximum value closer to the minimum value of 0.5 or less, the intensity of the output light at the timing when the frequency of the output light reaches the upper and lower limits Has an optimization function that makes the value instantaneously larger than the minimum value. Such intensity modulation means can be incorporated in any of the first to fourth embodiments.

  FIG. 20 is a simulation result of the output Brillouin spectrum of the BOCDA system when the optimized intensity modulation as shown in FIG. 19 is employed. FIG. 20A shows the output spectrum of the original “Half” intensity modulation and the optimized “Half” intensity modulation shown in FIG. 19 when the zero-distortion portion is the measurement point. In the optimized intensity modulation, the Lorentz spectrum corresponding to the measurement point can be clearly confirmed in both the case of “Full” and “Half”. On the other hand, FIG. 20B shows a spectrum when a place where distortion is applied is taken as a measurement point. In the optimized “Full” or “Half” intensity modulation, even if the distortion becomes large, the Lorentz type is shown. It can be seen that the spectrum is not buried in the Fuji-type background spectrum. In other words, with the optimized intensity modulation, the Lorentz spectrum peak can be made larger than the background spectrum peak even when the measured optical fiber has little or no distortion, and the measured optical fiber Regardless of the amount of strain on the FUT, accurate strain measurement is possible.

Furthermore, FIG. 21 is a simulation result in which the ratio of the spectrum of the measurement point and the height of the Mt. Fuji spectrum is plotted when the optimized intensity modulation as shown in FIG. 19 is employed. FIG. 21 (a) shows the original “Half” intensity modulation and the optimized “Half” intensity modulation shown in FIG. 19, respectively, but the Brillouin frequency corresponding to a position with little (or no) distortion. It can be seen that the SNR of the optimized “Half” intensity modulation is improved by the shift amount of ν _B. In addition, even with the shift amount of the Brillouin frequency ν _B corresponding to a position where the distortion amount is large, the SNR of the optimized “Half” intensity modulation is ensured to be 1 or more, and accurate measurement is possible.

Similarly, FIG. 21 (b) shows the original “Full” intensity modulation and the optimized “Full” intensity modulation shown in FIG. 19, respectively. Regardless of the shift amount of the frequency ν _B, an SNR sufficiently larger than 1 is maintained, and the position can be accurately measured from a small distortion to a large distortion. Thus, it can be seen that a waveform obtained by adjusting the “Half” or “Full” intensity modulation waveform as shown in FIG. 19 is an actually excellent intensity modulation waveform.

  In addition, this invention is not limited to the said Example, It can change in the range which does not deviate from the meaning of this invention. For example, frequency modulation in the detailed description of the invention includes phase modulation techniques. Further, as long as the light source can output frequency-modulated light, light by a technique other than the semiconductor laser may be used. Further, since the speed and amplitude in frequency modulation of the semiconductor laser (laser diode) 3 included in the light source 1 are limited, further improvement can be achieved by using the light source 1 with better modulation characteristics.

  The BOCDA method according to the present invention has improved the spatial resolution limit by a factor of 100 compared to the conventional method, and has improved the measurement speed by a factor of 10,000. It is the only technology in the world that has a random access function for location. For this reason, this technology is attracting attention as a neural network for materials and structures that can understand pain in a wide range of fields such as civil engineering / construction, aviation / space, nuclear energy / energy, transportation / transportation. The present invention can be expected to further improve performance and accelerate practical application.

It is a block diagram which shows the structure of the optical fiber characteristic measuring apparatus in 1st Embodiment of this invention. It is a block diagram which shows the structure of the optical fiber characteristic measuring apparatus in 2nd Embodiment of this invention. It is a block diagram which shows the structure of the optical fiber characteristic measuring apparatus in 3rd Embodiment of this invention. It is a block diagram which shows the structure of the optical fiber characteristic measuring apparatus in 4th Embodiment of this invention. It is explanatory drawing which shows the structure of the to-be-measured optical fiber FUT used as an experiment example of the apparatus of FIG. In the experimental example of the apparatus of FIG. 1, (a) is a graph of measurement results indicating the power spectrum of each modulated light, and (b) is a modulation voltage indicating a conventional frequency modulation waveform and the modulated light (IM1). It is a graph which shows the transmittance | permeability of the light intensity modulator corresponding to a power spectrum. 2 is a graph of Brillouin gain spectra measured in the DSF unit and the SMF unit using each modulated light (No IM, IM1, IM2, IM3) in the experimental example of the apparatus of FIG. FIG. 2 is a graph of Brillouin gain spectra measured in the DSF unit and the SMF unit when intensity modulation is not performed (No IM) and when optimum intensity modulation is performed (IM3) in the experimental example of the apparatus of FIG. . FIG. 2 is a graph of Brillouin frequencies measured by each DSF section when no intensity modulation is performed (No IM) and when optimum intensity modulation is performed (IM3) in the experimental example of the apparatus of FIG. It is explanatory drawing which shows the structure of the to-be-measured optical fiber FUT used in another experiment example of the apparatus of FIG. In another experimental example of the apparatus of FIG. 1, it is a graph which shows the power spectrum when not performing the conventional intensity modulation (No IM) and when applying the optimum intensity modulation (with IM). In another experimental example of the device of Fig. 1, Brillouin measured in the DSF and SMF sections using the conventional intensity modulation (No IM) and the optimum intensity modulation (with IM). It is a graph of a gain spectrum. FIG. 4 is a graph of Brillouin frequency measured in each DSF section when optimum intensity modulation is performed (with IM) in another experimental example of the apparatus of FIG. 1. 5 is a graph showing a frequency modulation waveform and a spectrum shape when the output light is frequency-modulated with a sine wave-like repetitive waveform and when the output light is frequency-modulated with a repetitive waveform other than a sine wave. (A) is a graph showing a normalized Brillouin gain spectrum shape when the output light is frequency-modulated with a sinusoidal repetitive waveform and when the output light is frequency-modulated with a repetitive waveform other than a sine wave; (B) is a graph which shows an absolute value Brillouin gain spectrum shape. It is a simulation result of the BOCDA system at the time of performing only frequency modulation, (a) is a graph which each shows the frequency modulation waveform and intensity | strength modulation waveform of light source output light, (b) is the time average spectrum of light source output light. It is a graph which shows a shape, (c) is a graph which shows an output spectrum shape, (d) is a graph which shows the relationship between a Brillouin frequency shift amount and SNR. FIG. 7 is a simulation result of the BOCDA system when there is intensity modulation accompanying frequency modulation, where (a) is a graph showing the frequency modulation waveform and intensity modulation waveform of the light source output light, and (b) is the light source output. It is a graph which shows the time average spectrum shape of light, (c) is a graph which shows an output spectrum shape, (d) is a graph which shows the relationship between a Brillouin frequency shift amount and SNR. It is a simulation result of the BOCDA system when intensity modulation is performed in synchronization with frequency modulation, where (a) is a graph showing the frequency modulation waveform and intensity modulation waveform of the light source output light, and (b) is the light source output. It is a graph which shows the time average spectrum shape of light, (c) is a graph which shows an output spectrum shape, (d) is a graph which shows the relationship between a Brillouin frequency shift amount and SNR. This is a BOCDA system simulation result when intensity modulation is performed in synchronization with frequency modulation to leave some intensity on both sides of the time-average spectrum, showing the frequency modulation waveform and intensity modulation waveform of the light source output light, respectively. It is a graph. FIG. 20 is a graph showing output spectrum shapes in each of the original intensity modulation in FIG. 18 and the optimized intensity modulation shown in FIG. 19. 20 is a graph showing the relationship between the Brillouin frequency shift amount and the SNR in each of the original intensity modulation in FIG. 18 and the optimized intensity modulation shown in FIG. It is the schematic explanatory drawing which showed typically the correlation peak in the to-be-measured optical fiber in a prior art example. It is the graph which showed in principle the Brillouin spectrum shape when not giving distortion to a correlation position, and when giving distortion to a correlation position.

Explanation of symbols

1 Light source (light source part)
4 Light intensity modulator (Intensity modulation means)
31 Probe light generation means
32 Pump light generation means
33 Measuring means
41 Optical filter (Intensity modulation means)
51 Signal generator (intensity modulation means)
FUT optical fiber to be measured

Claims (12)

A light source unit that outputs frequency-modulated light;
A probe light generating means for shifting the output light from the light source unit and making it incident as a probe light from one end of the optical fiber to be measured;
Pump light generating means for causing the output light from the light source unit to be incident as pump light from the other end of the optical fiber to be measured;
Measuring means for detecting a Brillouin gain of the probe light emitted from the optical fiber to be measured while sweeping a frequency difference between the pump light and the probe light, and measuring characteristics of the optical fiber to be measured. In the optical fiber characteristic measuring device,
An optical fiber characteristic measuring apparatus comprising intensity modulation means for modulating the intensity of the output light in synchronization with frequency modulation of the light source unit.   The intensity modulation means brings the intensity closer to the maximum value as the frequency of the output light approaches the fluctuation center, and approaches the minimum value as the frequency of the output light approaches the upper limit and the lower limit. The optical fiber characteristic measuring apparatus according to claim 1, wherein:   3. The optical fiber characteristic measurement according to claim 2, wherein the intensity modulation means sets the intensity of the output light to a value larger than the minimum value at a timing when the frequency of the output light reaches the upper limit and the lower limit. apparatus.   The optical fiber characteristic measuring device according to any one of claims 1 to 3, wherein the intensity modulation means is constituted by a light intensity modulator.   The optical fiber characteristic measuring apparatus according to any one of claims 1 to 3, wherein the intensity modulating means is configured by an optical filter.   The optical fiber according to any one of claims 1 to 3, wherein the intensity modulation means is configured by a signal generator that frequency-modulates output light from the light source unit with a repetitive waveform other than a sine wave. Characteristic measuring device. The frequency-modulated light of the light source unit is frequency-shifted, and incident as probe light from one end of the optical fiber to be measured.
The light frequency-modulated by the light source unit is incident as pump light from the other end of the measured optical fiber,
An optical fiber characteristic measuring method for detecting a Brillouin gain of the probe light emitted from the optical fiber to be measured while sweeping a frequency difference between the pump light and the probe light, and measuring characteristics of the optical fiber to be measured In
An optical fiber characteristic measuring method, wherein intensity modulation is performed on the output light in synchronization with frequency modulation of the light source unit.   The intensity modulation applied to the output light brings the intensity closer to the maximum value as the frequency of the output light approaches the fluctuation center, and the intensity is increased as the frequency of the output light approaches the upper limit and the lower limit. The optical fiber characteristic measuring method according to claim 7, wherein the optical fiber characteristic measuring method is close to a minimum value.   9. The intensity modulation applied to the output light is to make the intensity of the output light larger than a minimum value at a timing when the frequency of the output light reaches an upper limit and a lower limit. Optical fiber characteristics measurement method.   The optical fiber characteristic measuring method according to any one of claims 7 to 9, wherein the intensity modulation applied to the output light is performed by a light intensity modulator.   The optical fiber characteristic measuring method according to any one of claims 7 to 9, wherein the intensity modulation applied to the output light is performed by an optical filter.   The intensity modulation applied to the output light is performed by a signal generator that frequency-modulates the output light from the light source unit with a repetitive waveform other than a sine wave. An optical fiber characteristic measuring method described in 1.